Electron tube with a cold emissive cathode



Sept. 24, 1963 w. J. CHOYKE ETAL ELECTRON TUBE WITH A com EMISSIVECATHODE Filed Jan. 15, 1959 Anode Electrons WITNESSES:

'2 Sheets-Sheet 1 Fig.3.

INVENTORS Sept. 24, 1963 w. J. CHOYKE ETAL 3,105,156

ELECTRON TUBE WITH A COLD EMISSIVE CATHODE Filed Jan. 15, 1959 2Sheets-Sheet 2 Electrons Fig.5. 3

C 2 6 C 3 l i 2 i0" i0" |o" 10' I 10- Electron Emission Current- AmpsVacuum Level Electron Affinity T --Conduction Band k Fig 6, g? Bond Gapor Energy Gap Distance Through Crystal United States Patent 3,105,166ELECTRON TUBE WITH A COLD EMISSIVE CATHGDE Wolfgang I. Choyke,Wilkinshurg, and Lyle A. Patrick,

Penn Hills, Pa, assignors to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 15, 1959, Ser.No. 787,011 7 Claims. (Cl. 313-310) It is another object to provide asmall area electron source of low beam current.

It is another object to provide a cathode which omrates at roomtemperature and does not depend on thermionic emission. I

It is another object to provide an improved cathode in which theresponse is substantially instantaneous, thereby enabling one tomodulate the beam directly and at a high frequency rate.

It is another object to provide an improved cathode that does notrequire complicated activation techniques.

It is another object to provide a cathode which will operate at lowtemperatures.

It is another object to provide a cathode having a substantially longerlifetime than present thermionic erm'ssive-type cathodes.

These and other objects are effected by our invention as will beapparent from the following description taken in accordance with theaccompanying drawing throughout which like reference characters indicatelike parts, and in which:

FIGURE 1 is a schematic showing of a vacuum tube incorporating ourinvention;

FIG. 2 is an enlarged view of the cathode shown in FIG. 1;

FIG. 3 is an illustration of a modified cathode in accordance with theteachings of our invention;

FIG. 4 is an illustration of another modification of a cathode inaccordance with the teachings of our invention;

FIG. 5 is a curve indicating the electron emission plotted againstjunction current of a cathode embodying the teaching of our invention;and

FIG. 6 is a showing of simple energy band diagram for a semiconductorcrystal.

Referring in detail to FIG. 1 there is shown an electron device and morespecifically a diode-type receiving tube incorporating a cathode inaccordance with the teaching of our invention. The diode includes avacuumtight container 12 which may be of any suitable material, such asglass. A cup-shaped anode member 14 of a suitable material such asInconel, which is an alloy of about 79.5% Ni, 13.0% Cr, 6.5% Fe and'lesser amounts of Mn, Si, C and Cu, is positioned within the envelope'12. The anode 14 may be supported by a lead member 16 sealed throughthe top of the container 12 to which a positive voltage is applied by abattery 16. A cathode 20 is positioned in the lower portion of theenvelope 12 near the opening in the cup-shaped anode 14.

The cathode 20 consists of a body 22 of suitable semiconductive materialmounted on a support plate or elecat room temperature.

trode contact 24 of suitable materail such as Inconel. A lead 26 isconnected to the plate 24 and sealed through the lower portion of thecontainer 12-to support the oathode 20 within the container 12. The lead26 is connected to the negative terminal of a battery 28. A sec ondcontact 30 is provided to the cathode 20 which in the specificembodiment shown consists of a tungsten wire contact. The contact 30contacts the upper surface of the body 22 of material and is connectedby a lead-in 31 to the exterior of the envelope 12 to the positiveterminal of the battery 28.

The body 22 of material, as shown in detail in FIG. 2,

consists of a layer 32 of P-type material in contact on one surface withthe base contact 24- and a layer 34 of N-type material adjacent theother surface =P-type layer with an intermediate junction region 36. Thecontact member 30 is in contact with the N-type layer 34. By providing apotential of from 7 to 40 volts by means of the battery 28 across thebody 22 of material and collecting potential of volts to the anode 14by-the battery 16, an electron current was obtained which ranged from10- to 10' amperes.

The cathode body or wafer 22 in one specific emb'odi ment was siliconcarbide formed with a thickness of the layer 34 of N-type material ofless than 10 microns and the layer 32 of P-type material of a thicknessof about 1000 microns. The silicon carbide junction cathode may beprepared in several methods, one of which is disclosed in an articleentitled Electrical Contacts to'Silicon Carbide by R. N. Hall in theJune 1958 issue of the Journal of Applied Physics. Another method ofpreparing the silicon carbide junction is disclosed in a copendingapplication Serial No. 738,631,. filed May 29, 1958 and now Patent2,937,323, entitled Fused Junctions in Silicon Carbide, by L. I. Krokoet a1.

One specific method of fabricating the P-N junction in the siliconcarbide is to fuse small pellets of silicon containing a few percentboron to an N-type silicon carbide crystal by heating to a temperatureof 2000 C. for a period of 1 minute and then allowing the material tocool at a rate of 20 C. per second. The resulting ingot can be thentreated with hydrofluoric acid and nitric acid so as to etch away mostof the silicon and leave a P-type layer on the N-type silicon carbidecrystal. Before insetting the cathode material into the vacuum tube, itis also found advantageous to submerge the material within a suitableetching solution, for example, hydrofluoric acid for one-half of an hourto insure that the surface is entirely clean. The material may then bewashed in alcohol, mounted in the tube and then baked in a vacuum of 10?millimeters of mercury at a temperature of 270 C. for a period oftwenty-four hours.

Although silicon carbide was used in the specific embodiment, othersemiconductor and insulator materials are suitable. It is necessary thatthe material have an energy difference between the vacuum level and thebottom of the conduction band, which is referred to as electronafiinity, equal to or less than about the band gap. The band gap isdefined as the energy difference between the bottom of the conductionband and the top of the valence hand. These terms are also defined inthe energy band diagram shown in FIG. 6. In the case of silicon carbide,the electron atfinity is about 4 electron volts and the band gap ofsilicon carbide is about 2.86 electron volts We have found that thematerial must have electron affinity less than three halves of the bandgap of material. Such a relationship is most likely to'be found inmaterials having a large band gap, by which we mean a band gap of morethan 2 electron volts. Silicon, for example, which has been found to bea poor emitter, has a band gap of the order of 1 electron volt and anelectron afiinity of 3 electron volts. It is, therefore, important thatthe material used have an electron afiinity'less than the hole-electronpair production threshold which is of the order of 1.5 times the bandgapof the material. Other materials which may provide large band gapelectron sources with suitable junctions and which will serve asexternal electron emitters are All, AuCs, Gal ZnS, ZnSe, ZnO, NiO, A1TiO and diamond. V I

We have measured the electron emission current from the reverse biasedP-N junction in silicon carbide, and the emission there ranged from 10-to 10- amperes. In each case, the emission depended strongly on themethod of preparing the sample. It was found to be important that thesamples be heated at 270 C. in a vacuum for several hours. By applyingthe reverse bias to the silicon carbide junction, a breakdown radiationoriginating in small blue spots about 1 micron in diameter was obtained.These blue spots are observed both within the junction area and at itsperiphery. It is believed that electrons are emitted primarily fromthose spots which are very near the crystal surface. The cathode wasplaced in a holder in such a way that the tungsten wire contact Wasplaced on the highly conducting N-type surface which has a thickness ofthe order of microns. It is at this surface that the blue spots appear.It was found after first applying the reverse voltage to the junctionthat the electron emission current was observed to increase severalorders of magnitude during the first few minutes. Subsequently, theelectron emission response to reverse bias was instantaneous andreproducible Within a factor of 2 even after a long period of time. Itwas found that these so-called incubation periods are longer in thosesamples which were not baked in air or a vacuum. It is possible that asurface change caused by electron emission itself may be the reason forthe great increase ofemission during the incubation period. Becausebreakdownoccurs at the junction at small spots, the reversecharacteristic is too soft to define a specific breakdown voltage.

In FIG. 5, there .is illustrated the electron emission from a siliconcarbide junction plotted against the junction current. The reversevoltages range from 7 to 38 volts. It was found that the breakdownradiation first appears to a dark adapted eye at a single peripheralspot when the junction current is about the same as that at whichelectron emission begins. Other spots appear as the current is increaseduntil about thirty were visible at the highest reverse currents. Perhapsonly a few of these-contribute an appreciable number of electrons. Thetotal area ofall spots was estimated to be 10- centimeter square andhence the maximum emission current density was about 1 ampere per squarecentimeter. The sample was also tested to show that the electron currentwas not caused by breakdown radiation through a photoemission mechanism.It was found that'several samples emitted more electrons than photons.,Also, an oxide film on the surface would prevent electron emission but,did not noticeably affect light emission. It was noticed .that heatingthe sample in air at 800 C. for one hour also suppressed electronemission probably because of the formation of an oxide layer. At thereverse currents utilized, there was definitely very little, if any,heating of the silicon carbide crystal which might result in thermionicemission.

It isnecessary to provide a high internal electric field in thematerial, to obtain energetic electrons. The term energetic electronsmeans those having a greater energy than the electron affinity of thematerial. In this embodiment the high electric field was obtained by theformation of a P-N junction within the siliconcarbide. It is alsonecessary that the junction be close to the emission surface. In thecase of silicon carbide, the material between the junction and thecrystal surface should be less than '10 microns and, in general, lessthan 50 microns in thickness. It is also believed that the electronemission can be enhanced by the adsorption of a monomolecular layer ofalkali metal, such as cesium, at the junction so as to further reducethe electron afiinity of the material. Therefore, in those cases wherecopious external emission is more important than stability, an alkalimetal monolayer may be adsorbed on a suitable wide gap material. Thisadditional activation must be carried out under high vacuum conditions.Other materials that might be suitable for depressing theelectronatfinity are Ca, Sr, Ba, Ce and Th.

In PEG. 3, a modified cathode is illustrated in which a thin layer 44}of N-type material of less than 50 microns is provided on the surfacefacing the anode, and the electrode connection to the N-type layer 4% ismade by an annular conductive ring 42 of a material such as plati numaround the outer periphery of the N-type surface surrounding area may bemasked oil by providing. a

coating such as a suitable synthetic alkyd resin available under theproprietary name Glyptal.

While we have shown our invention in only a few formsjit will be obviousto those skilled in the art that it is not so limited, but issusceptible of various other charges and modifications without departingfrom the spirit and scope thereof.

We claim as our invention:

1. 'An electron tube comprising an electron emissive source, acollecting electrode for collecting electrons emitted from said electronemissive source, said electron emissive source comprising a'semiconductive body with a bulk material having an electron affinityless than three halves of the band gap of the material, means forproviding a high internal electric field in said body of material suchthat electrons are excited to energies greater than the electronatfinity of said material, and means 'for establishin a potentialbetween said electron 'emissive source and said collecting electrode forderiving an electrieal output from said electron device.

2. An electron emiss'ive cathode comprising a body of semiconductivesilicon carbide, said body of semiconductive silicon carbide including aP-N type junction, means for providing a reverse bias on said junctionto providea high electric field in the region of said junction such thatsaid electric field is of sufiicient value to excite conduction bandelectrons in the region of said junction to energies greater than theelectron afiin'ity of said semiconductive silicon carbide such thatelectrons are emitted from the surface of said cathode near saidjunction.

3. An electron emissive cathode comprising a semiconductor body with asemiconductive bulk material having an electron affinity less than threehalves-of the band gap of the material, said semiconductive bulkmaterial including a P-N type junction located near an exposed surfaceof the N-type region, means for providing a reverse bias on saidjunction to provide a high electric field in the region of said junctionsuch that said electric field is of stnficietnt value to exciteconduct-ion bandelectrons in the region of said junction to energiesgreater 5. An electron device comprising a collecting electrode and anelectron emissive source, said electron emissive source comprising awafer of silicon carbide, said wafer including a layer of P-typeconductivity and a layer of N-type conductivity separated by a junctionregion, said N-type layer facing said collecting electrode and having athickness of less than microns, means for establishing a reverse biasacross said junction to provide a high electric field in the region ofsaid junction such that said electric field is of suflic-ient value toexcite conduction band electrons in the region of said junction toenergies greater than the electnon afdnity of silicon carbide such thatelectrons are emitted from the surface of said N- type layer facing saidcollecting electrode.

6. An electron tube comprising an electron emissive source, a collectingelectrode for collecting electrons from said electron emissive source,said electron emissive source comprising a body of material selectedfrom the group consisting of silicon carbide, aluminum phosphide,gold-cesium, gallium phosphide, zinc sulfide, zinc selenide, zinc oxide,nickel oxide, aluminum oxide, titanium dioxide and diamond, means forproviding a high internal electric field in said body of material suchthat electrons are excited to energies greater than the electronafiinity of said material to cause emission of said excited electronsfrom said body of material, and means for establishing a potentialbetween said electron emissive source and said collecting electrode tocollect said excited electrons for deriving an electrical output fromsaid electron device.

7. An electron device comprising a collecting electrode and an electronemissive source, said electron emissive source comprising a wafer with*a semiconductive bulk material having an electron affinity of less thanthree halves the band gap of the material, said bulk material includinga layer of P-type material and a layer of N-type material separated by ajunction, said N-type layer having an exposed surface facing saidcollecting electrode and having a thickness of less than microns, meansfor establishing 'a field across said junction of sufiicient value toexcite conduction band electrons in the region of said junction toenergies greater than the electron aflinity of said bulk material sothat electrons are emitted from the exposed surface of said N-type layerfacing said collecting electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,592,683 Gray Apr. 15, 1952 2,719,241 Coltman Sept. 27, 1955 2,735,049DeEor est Feb. 14, 1956 2,842,706 Dobischek et al. July 8, 19582,879,424 Garbuny Mar. 24, 1959 2,938,141 Choyke May 24, 1960 2,960,659Burton Nov. 15, 1960 OTHER REFERENCES Electron Emission From AvalancheBreakdown in Silicon, by I. A. Burton, pages 1342, 1343, PhysicalReview, vol. 108, No. 5, December 1, 1957.

1. AN ELECTRON TUBE COMPRISING AN ELECTRON EMISSIVE SOURCE, A COLLECTINGELECTRODE FOR COLLECTING ELECTRONS EMITTED FROM SAID ELECTRON EMISSIVESOURCE, SAID ELECTRON EMISSIVE SOURCE COMPRISING A SEMICONDUCTIVE BODYWITH A BULK MATERIAL HAVING AN ELECTRON AFFINITY LESS THAN THREE HALVESOF THE BAND GAP OF THE MATERIAL, MEANS FOR PROVIDING A HIGH INTERNALELECTRIC FIELD IN SAID BODY OF MATERIAL SUCH THAT ELECTRONS ARE EXCITEDTO ENERGIES GREATER THAN THE ELECTRON AFFINITY OF SAID MATERIAL, ANDMEANS FOR ESTABLISHING A POTENTIAL BETWEEN SAID ELECTRON EMISSIVE SOURCEAND SAID COLLECTING ELECTRODE FOR DERIVING AN ELECTRICAL OUTPUT FROMSAID ELECTRON DEVICE.